Outside shot... A shot of one of two astronauts carrying out repairs on the broken equipment cooling system. Picture: NASA

If not, the Christmas Day outing would be the first since 1974, when a pair of NASA astronauts "stepped outside the Skylab space station to retrieve film from a telescope and photograph Comet Kohoutek," the US space agency said.

The urgent spacewalks were called for this week due to a faulty valve that caused a partial shutdown in the system that regulates equipment temperature at the space station.

Engineers tried to fix the problem from the ground, but eventually decided they needed to replace the ammonia pump.

The six-man crew was never in danger, but NASA wanted to repair the problem sooner rather than later, agency officials said.

The spacewalks meant the first regular commercial cargo supply mission by Orbital Sciences' Cygnus craft, which had been planned for earlier in the week, was postponed. It will now fly no earlier than January 7.

NASA also had to rig up some last-minute contingency gear inside the US-made spacesuits, which have not been used since a helmet water leak nearly drowned a European astronaut in July.

Now the helmets carry an extra absorption pad and a snorkel inside, just in case.

The investigation into the cause of the helmet leak is ongoing. Hopkins will be wearing the suit that had the problem, though its inner water pump has been replaced.

During the initial walk, NASA said it would check with the astronauts throughout the day to make sure they were not feeling any "squishy" sensation behind their necks, which might indicate a water buildup and would cut short the spacewalk, space agency commentators said.

The first check, 31 minutes into the outing, was clear, with astronauts reporting dry conditions and no problems in the spacesuits or gloves, NASA said.

The astronauts also trained for the emergency spacewalks by watching video of a previous outing to replace an ammonia pump in 2010.

With each astronaut sporting a helmet camera, NASA TV showed the operations in real-time from the perspective of the spacewalkers.

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